The present invention relates to materials and methods for eliminating or reducing the likelihood of interference between reactor components, for example, a flow channel and an associated control blade, in a nuclear reactor core so that the shutdown and operation of the plant can be performed more efficiently and safely. Channels used in the nuclear energy industry are typically formed from the zirconium alloys, such as Zircaloy-2 and Zircaloy-4, which have relatively high general corrosion resistance, but which are also known to be susceptible to bowing. It has been shown that interference between a flow channel and an associated control blade can result from deformation, specifically bowing, of the flow channel as a result of non-uniformity, particularly side-to-side differences, in the irradiation-induced growth or the hydrogen content of the flow channel material.
These differences in the hydrogen content have recently been discovered to be attributable at least in part to corrosion differences that are related to localized “shadow” corrosion resulting from enhanced corrosion induced by the close proximity of the dissimilar materials of the control blade and certain regions on two of four adjacent channel sides. The corrosion of zirconium alloys in aqueous environments generates hydrogen as a reaction product. A portion, commonly referred to as the pickup fraction, is absorbed into the alloy and can induce a dimensional change, specifically dilation, of the zirconium alloy component. Bowing from all sources is additive, and therefore the adverse effect of shadow corrosion bowing is exacerbated when irradiation growth, a phenomenon resulting from exposure to differential radiation gradients is also occurring in the control blade and flow channel components.
The service life of a boiling water reactor (BWR) fuel channel is generally limited by the ability of the channel to resist longitudinal bowing, creep and corrosion. Bowing has been attributed to the alloys' irradiation growth properties wherein neutron irradiation tends to increase the length of a zirconium-based alloy in a direction coincident with the rolling direction of the irradiated component. This phenomenon is known to be a strong function of fast neutron fluence and the persistent presence of a gradient which can be present transversely through a channel resulting from a higher fast neutron flux level at the center of the core than towards the perimeter of the core.
Accordingly, in the presence of a persistent flux gradient, there is a tendency for one side of a fuel channel to “grow” more than the opposite side of the channel and causing the channel to develop a longitudinal bow. While the longitudinal bow may not be enough to alter the flow or mechanical properties of the channel significantly, the bowing can be sufficient to increase the likelihood of interference with the free movement of its adjacent control blade, which must be capable of being moved along or stroked relative to the channel for controlling the nuclear reaction. Eventually, some channels may develop an unacceptable degree of bowing and will need to be replaced.
The degree to which a Zircaloy component will exhibit irradiation growth is a function of the alloy composition and its metallurgical state, specifically the crystallographic texture of the component and the extent of any retained cold work or residual stress, which can enhance irradiation growth. One method for reducing the tendency for zirconium alloy fuel channels to bow is a metallurgical treatment applied to the zirconium alloy to reduce its propensity for irradiation growth. One such method includes the combined steps of heat treating, warm forming and thermal sizing a Zircaloy alloy to yield a fuel channel that exhibits a random texture, reduced residual stress and hence reduced irradiation growth. Another method for reducing channel bow is to form the fuel channels from a zirconium alloy that exhibits inherently reduced irradiation growth.
One class of zirconium alloys that is promising for low irradiation growth applications are those containing niobium. One such alloy has an approximate composition of, in weight percent, about 1.2% tin (Sn), about 1.0% niobium (Nb), and an iron (Fe) content of about 0.34-0.40%, with the balance comprising essentially zirconium (Zr) and incidental impurities. One such alloy, which has an exemplary composition of Zr0.974Sn0.012Nb0.01Fe0.004, is identified within the art as Alloy E635. General Electric has developed and continues to refine a class of Zr—Nb alloys, referred to as NSF, that have a composition of, in weight percent, about 0.5-1.2% tin, about 0.6-1.2% niobium and about 0.2-0.5% iron with the balance essentially zirconium and incidental impurities.
The term NSF is a somewhat generic term reflecting the presence of niobium (Nb), tin (Sn) and iron (Fe) as the primary alloying metals combined with the zirconium. Another variation has been employed by the Westinghouse Electric Corporation under the name ZIRLO that utilizes a composition of, in weight percent, about 1.0% tin, about 1.0% niobium, and about 0.1% iron, with the balance essentially zirconium and incidental impurities, and was developed primarily for use as fuel cladding in pressurized water reactors (PWR) for improving upon the corrosion resistance provided by Zircaloy-4 in PWR environments.
However, it is well known that the corrosion resistance of a particular zirconium alloy is highly dependent on the type of reactor environment in which it is used, and that the relative corrosion resistance, hydrogen pickup, and shadow corrosion bowing resistance of a particular alloy composition within a given environment cannot be accurately predicted based on its composition. In the case of the Zr—Nb alloys, testing has indicated that these alloys generally have a lower resistance to corrosion than the Zircaloy alloys when employed in boiling water reactor (BWR) environments.
While the zirconium alloys and their processes have provided advances in resistance to corrosion and longitudinal bowing, further improvements are desired to yield nuclear reactor components, such as fuel channels, that are capable of a longer service life, particularly in boiling water reactor environments, and particularly when exacerbated by the newly discovered phenomenon of shadow corrosion-induced bowing.